The identity and diversity of arbuscular mycorrhizal (AM) fungal symbionts strongly affect the functioning of mycorrhiza, but little is still known about the functional relevance of the individual taxa abundances within AM fungal communities. We hypothesized that proportions of AM fungal taxa influence mycorrhizal benefits to the host, and that a community with spontaneously established ratios of AM fungal species is more beneficial than communities with artificially manipulated ratios. Medic (Medicago truncatula) was inoculated with synthetic AM fungal communities composed of five fungal species in different proportions, and a ‘functionally optimized’ community generated by previous co-cultivation of the five AM fungi. The composition of the communities was monitored along with the host plant responses to mycorrhiza in three sequential harvests.

Most of the artificial AM fungal communities differed compositionally and functionally from the presumably ‘functionally optimized’ community, which indeed promoted plant growth to the greatest extent. The higher ability to promote plant growth was partly explained by higher intraradical fungal biomass, but functional differences between the communities were also related to the abundances of certain AM fungal species. Thus, the experiment demonstrated functional relevance of species' abundances within AM fungal communities. The observed ‘functional optimization’ of the AM fungal community is discussed in context with the host plant's and AM fungal species' traits.

丛枝菌根（AM）真菌共生菌的身份和多样性强烈影响菌根的功能，但对AM真菌群落中单个分类单元丰度的功能相关性知之甚少。我们假设AM真菌类群的比例会影响菌根对寄主的益处，并且具有AM真菌物种自发比例的社区比具有人工操纵比例的社区更有益。军医（Medicago truncatula）接种了由5种不同比例的真菌组成的合成AM真菌群落，以及通过先前5种AM真菌的共培养产生的``功能优化''群落。在三个连续的收获期中，对群落的组成以及寄主植物对菌根的反应进行了监测。

大多数人工AM真菌群落的组成和功能与假定的“功能最优化”群落不同，后者确实最大程度地促进了植物的生长。较高的促进植物生长的能力部分地由较高的自由基内真菌生物量解释，但是群落之间的功能差异也与某些AM真菌物种的丰度有关。因此，实验证明了AM真菌群落中物种丰度的功能相关性。结合宿主植物和AM真菌物种的性状讨论了AM真菌群落的“功能优化”。